This invention relates to photoflash firing circuits and more particularly to an improved photoflash sequencing circuit employing either logic level or pulse triggering.
Photographic flash systems in which a plurality of flash lamps are arranged as an array within a single disposable package are advantageous in that the arrayed lamps are oriented for providing illumination from each individual lamp without recourse to moving or rotating the flash lamps on their mounting, etc. Flash arrangements of this type generally require firing circuits which sequentially ignite the bulbs in a predetermined order from first to last in synchronization with successive exposures.
Such firing circuits generally depend upon the variation in lamp characteristics before and after firing to provide proper sequencing. Generally, flash lamps with which the sequencing circuit must operate include a glass envelope within which is sealed a quantity of combustible filamentary strand material in combustion supporting gas such that upon application of current to the terminals of the lamp, the igniting filament rapidly heats to the combustion point of the strand material and flash ignition ensues. During ignition, the filament is generally open circuited such that upon completion of ignition the lamp terminals are opened circuited and the switching circuit to which they are coupled witnesses an infinite resistance.
In one flash sequencing circuit available in the prior art, a plurality of lamp switching circuits are coupled in series with respective lamp terminals and intercoupled with a forward sequencing circuit which sequentially fires each bulb in succession and by passes open or previously fired bulbs. In this circuit, each lamp with its associated switching circuit is coupled in parallel across a power source. Interconnected to each switching circuit is a forward sequencing circuit which in response to an appropriate trigger pulse is configured to fire the next operable lamp in the array. In this arrangement, the sequencing circuit is triggered so as to successively fire each individual lamp circuit in accordance with a voltage pulse which exceeds the source voltage. Since it is often desirable to employ the full voltage of the camera battery for lamp ignition, this triggering arrangement requires that the camera exposure system provide a pulse in excess of source voltage. Many camera exposure systems do not readily provide an appropriate pulse voltage of this type. Hence, it is important to provide a photoflash firing circuit designed for low voltage triggering. Further, since firing circuits of this type may be advantageously constructed as a unitary semiconductor chip, it is also important to provide a basic chip having both triggering capabilities.
Consequently, it is a primary object of this invention to provide an improved photoflash sequencing circuit capable of being operated by a level logic signal.
It is another object of this invention to provide a photoflash sequencing circuit incorporating a logic gate triggering system.
It is still another object of this invention to provide a photoflash sequencing circuit incorporating both low level logic and pulse triggering capabilities.